Reflective and pearlescent flake pigments are used in decorative coatings to provide aesthetically pleasing effect because of both sparkle and "flop". "Flop" is a polychromatic effect whereby, due to the presence of the pigment, a lighter color is evident to the eye when the coated surface is viewed perpendicularly than when viewed at low angle. These variations emphasize the lines and contours of a three dimensional surface.
The weathering resistance of many decorative coatings can be adversely affected by the presence of a reflective pigment or pearlescent pigment. Many synthetic polymers used in coatings are degraded by exposure to the ultraviolet light present in sunlight. Whereas most pigments absorb ultraviolet light, reflective pigments scatter and reflect ultraviolet light throughout the polymer matrix of the coating, increasing its exposure to ultraviolet light. Pearlescent pigments absorb ultraviolet light and catalyze photo-degradation of the polymer matrix of the coating.
Reflective flake pigments are typically flakes of metal, such as aluminum, nickel, or steel. Pearlescent pigments are typically mica flakes coated with metal oxides. Certain of these materials are subject to attack by corrosive atmospheres, producing spotting and/or staining of the coating. Metals and metal oxides can enhance photodegradation of the polymer matrix. In addition, these materials may be difficult to disperse in the paint vehicle.